Sixty-three days after he stunned the world by resigning the papacy, Pope Emeritus Benedict XVI was flown by helicopter back to the Vatican, where he will live out the rest of his days.

His return heralds the start of an unprecedented chapter in the history of the Catholic Church, in which for the first time two Popes will live together within the walls of the tiny city state.

The 86-year-old, increasingly frail Pope Emeritus was flown by helicopter from Castel Gandolfo, the papal summer residence outside Rome where he has been living since renouncing the Seat of St Peter on Feb 28.

The 15-minute flight took him back over the rooftops and church spires of Rome, in a reversal of the historic journey he took on his last day as pontiff.

His white helicopter landed on a helipad in a far corner of the Vatican gardens, from where he was driven to the former convent, which has been converted into his retirement home.

There he was met by his successor, Pope Francis, 76, and the two men prayed together in a chapel adjoining the ex-convent.

"He is happy to be back at the Vatican ... where he intends to dedicate himself to the service of the Church, above all with prayer," the Vatican said in a statement.

The Mater Ecclesiae convent, situated on a hill within the city state, commands views of St Peter's Basilica, the Spanish Steps and other famous Rome monuments, and the distant Apennine mountains.

It is a 10 minute walk from Casa Santa Marta, the Vatican residence where Pope Francis has chosen to live since his election, having rejected the option of living in the much larger and more sumptuous papal apartments.

Vatican analysts have speculated that the presence of a "shadow Pope" could cause difficulties for Francis, with the risk that Vatican insiders unhappy with the radically different style of the new Pope could look to the Pope Emeritus for guidance and even form a rebel faction.

"Benedict almost certainly will be a point of reference for critics of Francis, especially in conservative circles. You can easily imagine them saying, 'Benedict wouldn't have done it this way,'" John Allen, a Vatican analyst with the US-based National Catholic Reporter, told Reuters.

But Vatican officials insist that the Pope Emeritus intends to devote the rest of his life to study and prayer and will not be making public pronouncements which could contradict those of his successor.

When he announced his decision to step down, the Pope Emeritus said he would live out his remaining days "hidden from the world".

The fact that the two Popes will share such a tiny territory will throw up some unusual scenarios, such as encounters in the Vatican gardens, chapels and libraries, although such meetings will take place hidden from the outside world.

The Pope Emeritus will live on the ground floor of the convent, which has undergone extensive renovations.

He will be aided by a small staff, including his personal secretary, Archbishop Georg Ganswein, nicknamed "Gorgeous Georg" for his good looks.

The Vatican insists that the Pope Emeritus does not have a serious illness or medical condition, but is simply slowing down as a result of his advanced age.

Not since the 13th century has a Pope had the chance to meet, much less live cheek-by-jowl with, his still living predecessor.

In 1294, Pope Celestine V became one of the very few Popes to resign, complaining that he had never wanted the role in the first place.

The former mountain hermit held discussions about his decision to quit with Cardinal Benedetto Gaetani, who succeeded him and took the name Boniface VIII.

The ex-Pope then fled Rome as quick as he could, anxious to resume his life as a cave-dwelling ascetic in the Abruzzo region of central Italy.

Unfortunately for him, Boniface VIII saw him as a threat, fearing that enduring loyalties to the former pontiff could provoke a schism.

He had Celestine arrested, hauled him back to Rome and imprisoned him in a castle at Ferentino, about 40 miles south-east of the city. Celestine died there ten months later, in May 1296.